This invention relates to a system and method for logging geological formation traversed by a borehole and more particularly to measuring formation resistivity through a cased borehole, wherein resistivity measurement is performed in a continuous, non-stop fashion.
Measurement of the formation resistivity has been a well-known method to determine presence of hydrocarbons in a formation traversed by boreholes. Typically, however, a borehole is cased shortly after drilling to provide structural integrity of the well. Consequently, the technique of resistivity-through-casing (RTC) was developed to measure the formation resistivity from within the cased well. A general problem of formation RTC measurements is the high electromagnetic attenuation due to the high conductivity of the casing material, since such material is typically a single-wall mild-steel pipe with a conductivity of the order of 106 S/m (resistivity is about 10xe2x88x926 xcexa9m). Galvanic RTC measurements provide one method capable of overcoming this problem due to electromagnetic attenuation.
There are several galvanic measurement methods, all of which have a common measurement principle: a known current is injected downhole into the casing and is returned through a surface electrode far from the wellhead. The casing leaks current into the formation, and the corresponding loss of current, being proportional to the local formation conductivity, can be determined by comparing the voltage drop across adjacent sections of the casing.
Important parameters of the galvanic method include, for example the characteristic length xcexL, which determines on what length scale most of the current has leaked into the formation. This parameter is approximately given by             ρ      R        ,
where xcfx81 is the average (global) formation resistivity and R is the casing resistance per meter. For typical values of xcfx81=10 xcexa9m and R=40 xcexcxcexa9/m, the characterisitic length xcexL≈500 m. The characteristic impedance Q, is yet another important parameter, given by Q=RxxcexL/2. Q also is equal to the potential developed at the injection point for a current of 1A. For example, R=40 xcexcxcexa9/m and xcexL≈500 m, then Q=10 mxcexa9; that is a current of 10A develops a voltage drop V0 of 100 mV between the injection point and a (infinitely) remote return electrode.
Generally, galvanic RTC measurement is performed in the following fashion. A current (typically I0=10A) is injected into the casing; this current splits evenly in all directions. Close to the injection point, assuming a local formation resistivity of xcfx81=10 xcexa9m, the pipe leaks current at a rate of dI=I0/2/xcexL=10 mA/m. This loss of current is proportional to the local formation conductivity and can be determined by comparing the voltage drop across adjacent sections of the pipe. Over the first 1 m section, for example, a voltage drop of 5Axc3x9740 xcexcxcexa9/mxc3x971 m=200 xcexcV can be measured. The next 1 m section sees 4.99Axc3x9740xcexcxcexa9Q/mxc3x971 m=199.6 xcexcV, the difference being 400 nV. Assuming that the exact casing resistance in both the intervals is known, one can determine the leakage current xcex94I over an interval xcex94z=1 m. The apparent local formation resistivity reading is approximately xcfx81a=kxcex94z V0/xcex94I, where k is a geometric parameter on the order of 1, which depends weakly on the average formation resistivity, the casing resistance and the casing radius.
The problem with this method is that the resistance of the casing is variable. In order to resolve 100 xcexa9m in xcfx81a, one needs to be accurate down to at least 40 nV in the difference voltage. With 5A passing through the casing, 40 nV are added or subtracted by a change of 8 nxcexa9 or 0.02%. The pipe is typically corroded and its diameter and resistance vary much more than that. As a result, the measurement of the resistivity must be done in a stop-and-go fashion. At every station the tool has to initially stop to determine the resistivity of casing, then it can determine the resistivity of the formation, and finally it can move to the next station.
Stop-and-go RTC measurement methods have been implemented in a tool developed by Baker-Atlas, a division of Baker Hughes Inc. Various realizations of the Baker-Atlas tool have been adapted in the industry. The underlining measurement principle of stop-and-go RTC tools is typically that shown in FIG. 1. A known current I0 is passed along the casing from an electrode A to an electrode B (the remote electrode at the surface). I0 is typically in the range of 5 to 10 amperes. The current has to leave the casing and traverse the formation in order to arrive at the surface electrode B. One-half of this current flows past the electrodes C, D, and E. These electrodes and the connected differential amplifiers register the voltage drop due to the casing resistance. If no formation current is present, the voltage drops C-D and D-E are equal, assuming equal pipe resistance in the intervals C-D and D-E. Current leakage, i.e., formation conductivity, is indicated by an imbalance between the voltage drops, which result in a net difference voltage Vout.
In practice, the pipe resistivities are unbalanced and a nulling cycle, shown in FIG. 2, is required to determine the pipe resistivity at the measurement point and to compensate for any offset voltages and gain differences in the amplifiers. During nulling, the current I0 is passed between electrodes A and F, a mode in which very little formation leakage occurs. The gain of one differential amplifier is adjusted until Vout becomes zero. This nulling operation is done at every new station. Once Vout, has been nulled, the tool must not move before the measurement mode, shown in FIG. 1, is completed. This in turn necessitates a stop-and-go operation between measurements.
In addition, the Baker-Atlas type tools generally exhibit strong boundary effects in the presence of inhomogeneities that approach the length scale xcexL. Under these conditions, the injected current no longer splits up evenly and the current portion that flows under the sensing electrodes C, D, E becomes unknown. For example, approaching an oil-water contact, the injected current would preferably flow into the direction of the water. Depending on the orientation of the tool, this increases or decreases the sensed voltage differences, causing a gross misreading of the local resistivity due to distant changes in large-scale conductivity.
Some of the shortcomings of the Baker-Atlas design have been recognized in the U.S. Pat. No. 5,075,626 to Vail (the xe2x80x9cVail patentxe2x80x9d). The Vail patent proposes to use two different frequencies: a lower one for the current traversing the formation and a higher one to only sense the casing resistance. The problem is that these two currents penetrate the casing to different skin depths (due to difference in frequencies) and experience different resistance.
Another problem of the Baker-Atlas designxe2x80x94supplying a large current over the wirelinexe2x80x94is addressed in the U.S. Pat. No. 5,510,712 to Sezginer (xe2x80x9cSezginerxe2x80x9d). In accordance with Sezginer, the RTC tool may optionally be powered efficiently to replace the surface based current supply with current sources in the tool. Sezginer eliminates the use of surface electrodes by deploying two opposing current loops, each extending over about 10 m of the casing. Between the two current loops, several voltage electrodes monitor the voltage drop due to current leakage into formation. This approach requires that, the tool be very long (at least 22 m). In addition, the sensed voltages are smaller by two orders of magnitude than in the tool-to-surface configuration because most of the current simply circulates on the casing and does not contribute to the measurement.
An alternative solution is proposed in U.S. Pat. No. 5,563,514 to Moulin, in which a Wheatstone Bridge is used as a sensing element. One leg of the bridge is formed by two sections of the casing, contacted by three voltage-sensing electrodes, and the other leg is realized by a xe2x80x9cpotentiometerxe2x80x9d-type circuit. In the first of two steps, a current is passed only along the casing and the potentiometer circuit adjusted to eliminate the net bridge voltage. Then, in a second step, during the actual measurement, a current is passed from the tool to the surface. The bridge becomes unbalanced. Balance is restored by injecting an additional current at a center electrode in proportion to the error voltage sensed by the bridge amplifier. The design is difficult to realize in practice since the voltages involved are very small to begin with and must be divided with great precision.
The above-described RTC tools require a stop-and-go logging process. In particular, at every measurement station the tool has to initially stop to perform a nulling cycle, which compensates for changes in casing resistivity, followed by determination of the resistivity of the formation. In other words single resistivity measurement requires at least two steps. Moreover, the tool exhibits strong boundary effects in the presence of inhomogeneities that approach the length scale xcexL. Under these conditions, the injected current no longer splits up evenly, and the current portion that flows under the sensing electrodes becomes unknown. For example, approaching an oil-water contact, the injected current would preferably flow into the direction of the water. Depending on the orientation of the tool, this increases or decreases the sensed local voltage differences, causing gross misreading of the local resistivity due to distant changes in large-scale conductivity.
Accordingly, it is one object of the present invention to provide a logging tool that directly measures formation resistivity through a cased borehole without preliminary computation of casing resistivity. It is another object of the present invention to provide method for RTC logging that enables direct measurement of formation resistivity by compensating, in a simple and effective manner, for variations in the casing resistivity. It is another object of the present invention to provide a RTC tool and method for continuous, non-stop resistivity logging in which the nulling cycle is eliminated so that the formation resistivity is measured directly. Yet it is another object of the present invention is to provide a RTC tool that is unaffected by the strong boundary conditions that may exist in the presence of heterogeneous formations.
The disclosed invention comprises methods and systems for continuous estimation of the formation resistivity through the casing of a borehole by eliminating the need to estimate the actual casing resistance by providing a real-time feedback control to eliminate current flow along the, usually, conductive casing in the vicinity of an injection point for the injection of a measuring current into the formation. This allows a direct estimation of the current injected into the formation along with the casing voltage used to drive the current into the formation. The preferred method for generating the feedback is to use the proportional integral derivative procedure to correct for both offsets and overcompensation. Preferably a digital signal processor generates the feedback and uses it to control bi-directional steerable current sources supplying one or more currents to the casing at various points. However, the illustrative preferred embodiment is not intended to limit the scope of the invention, and instead provides a workable example of the general principles. Thus, the following disclosure also teaches principles for the design of continuous formation resistivity measuring equipment and procedures in cased boreholes and similar challenging environments.
In contrast to the above-described RTC tools, the RTC tool in accordance with a preferred embodiment of the present invention is capable of directly measuring the resistivity of a formation traversed by a cased borehole without a prior nulling cycle in which changes to the casing resistivity are compensated for. To this end, the RTC tool of the present invention enables optional non-stop measurement of the formation resistivity at various measurement stations. Accordingly, resistivity measurements can be taken with a moving RTC tool to efficiently generate a moving RTC log.
The disclosed system and method may operate in a region of the borehole casing from which a measuring current is injected into the formation. This current may be measured directly along with the voltage of the casing at the point or region of injection. Current flow along the casing is reduced to within acceptable error in estimated formation resistivity by using one or more balancing or additional current sources that are adjusted in real-time to substantially eliminate current flow along the casing that may affect the estimation or injection of measuring current into the formation. As is apparent several measuring and balancing current sources may be used, although the preferred embodiment uses only two balancing and one measuring current sources. In the preferred embodiment, these current sources are arranged symmetrically about the measuring current injection electrode. In addition, there are measuring electrode pairs deployed in the regions where no current flow along the casing is desired to provide a sensor for detecting and correcting any such current flow.
The system and method of the present invention include improvements to the prior art stop-and-go mechanism for RTC measurement. For instance, the RTC measurement in accordance with the present invention does not require determination of the casing resistivity. Therefore, there is no need for xe2x80x9cnullingxe2x80x9d or calibration cycle so that the measurement of formation resistivity at various stations can be done in a continuous fashion. Consequently, RTC measurement can be done with a moving tool that is a foundation for a moving RTC log.
In one aspect of the invention, a system for measuring formation resistivity comprises at least two instrumental amplifiers, at least two analog-to-digital (A/D) converters, at least one digital signal processor (DSP) and plurality of bi-directional, steerable current sources regulated by the DSP.
In another aspect of the invention, a method for measuring formation resistivity comprises steps of injecting first, second and third currents, monitoring voltage drop across two regions formed between the injection points, and adjusting the first, second and third currents as to eliminate potential difference between the first and second regions.
In yet another aspect of the invention a preferred algorithm for operation of the DSP is disclosed, in particular for providing real-time feedback to the current sources. The algorithm, based on the proportional integral derivative (PID) procedures, not only corrects in proportion to the error, i.e., voltages due to current flow along the casing about the injection of the measuring point, it also provides a correction for removing offset by using cumulative corrections (the integral contribution) and the trend of the correction (the derivative contribution) to provide a robust real-time feedback. It should be noted that the DSP is intended to not only encompass hardwired signal processors, but also programmable and configurable processors to provide a tuned response suitable for various conditions and specifications.
The DSP performs the essential real-time control and signal processing activities. First, the DSP performs phase-coherent detection of the operating frequency on all input signals. Then, it conducts time-domain averaging of the received signals to increase their signal-to-noise ratio (SNR). Next, it detects any imbalance between the two voltage signals. In case any imbalance is detected, the DSP equalizes the potential differences in the region between the voltage-sensing electrodes by regulating via bi-directional, steerable current sources the amount of current injected through the current-injecting electrodes. In particular, the DSP performs PID and PWM algorithms that respectively compute amount of injected current needed to reach equilibrium and corresponding ON/OFF conditions for the current sources.
Additional aspects of the invention are described in further detail below.